The present invention generally relates to electrostatic capacitance type information signal recording discs and methods of manufacturing the electrostatic capacitance type information signal recording discs, and more particularly to an electrostatic capacitance type information signal recording disc to be played on a reproducing apparatus which reproduces recorded information signals as variations in electrostatic capacitance between the disc and an electrode of a reproducing stylus, and to a method of manufacturing such an electrostatic type information signal recording disc.
Recently, a system has been reduced to practice wherein a reproducing apparatus reproduces recorded information signals from an electrostatic capacitance type information signal recording disc. The information signals are recorded on the disc as variations in geometrical configuration such as formation of pits. The reproducing apparatus reproduces the recorded information signals from the disc as variations in electrostatic capacitance between the disc and an electrode of a reproducing stylus. This type of disc is used as a video disc which is recorded with video signals, an audio disc which is recorded with audio signals, and the like. In a certain kind of the audio disc, the disc is recorded with video signals related to still pictures in addition to the audio signals.
In any kind of electrostatic capacitance type information signal recording disc, a part of the disc containing the variations in geometrical configuration functions as an electrode which forms the electrostatic capacitance between the electrode of the reproducing stylus. Thus, this part of the disc is made of a conductive material. The conventional electrostatic capacitance type information signal recording disc is formed by heating and pressing, by use of stampers, a conductive resin composition which is obtained by admixing a conductive material such as carbon black powder into polyvinyl chloride resins.
However, according to the conventional electrostatic capacitance type information signal recording disc, the entire disc is formed from the conductive resin. Hence, even the central part of the disc in the thickness direction thereof which does not contribute to the signal reproduction, is formed from the conductive resin. But as is well known, the carbon black powder is relatively expensive, and the conductive resin is accordingly expensive. Therefore, the conventional disc which is formed from the conductive resin in its entirety is expensive, and there is a disadvantage in that no measures can be taken to reduce the manufacturing cost of the disc because the conductive resin is expensive.
When manufacturing the conventional disc, a tablet which is made of a conductive resin and has a size such that the diameter thereof is approximately 1/3 the diameter of the disc is prepared. This tablet is heated and pressed by a pair of stampers and is then rolled, so as to obtain a disc having a predetermined thickness and having the variations in geometrical configurations formed on the surface thereof. However, when the tablet (conductive resin) is heated and pressed by the stampers and rolled, the conductive resin is spread between the stampers. Hence, the stamping surfaces of the stampers are easily damaged by the spreading conductive resin which rubs the stamping surfaces. As a result, there is a disadvantage in that the serviceable life of the stampers is short.
In addition, it takes a certain time to prepare the tablet, and there is a disadvantage in that it is difficult to reduce the disc manufacturing cycle. Furthermore, in order to prepare uniform tablets, it is necessary to adjust a tablet forming apparatus which is provided with respect to each disc pressing (compression molding) apparatus which forms the disc by use of the stampers. Consequently, there is a disadvantage in that a troublesome operation is required to adjust the tablet forming apparatus.
On the other hand, a disc forming method in which the disadvantages described before that the disc is expensive because the entire disc is made from the conductive resin containing the expensive carbon black powder is eliminated, was proposed in a Japanese Laid-Open Patent Application No. 58-55224 (Japanese Patent Application No. 57-154470 filed Sept. 3, 1982). This previously proposed disc forming method comprises the steps of forming sheets which are made from a conductive plastic containing the carbon black, positioning the sheets with respect to the stampers within a metal die which is used for compression molding, placing a core material which is made from a non-conductive plastic and is in the form of a dumpling so that the core material may be subjected to the compression molding between the sheets, and forming a disc having such a sandwich type construction that the surface parts of the disc are conductive and the central part of the disc is non-conductive.
According to the disc forming method proposed in the above Laid-Open Application, the quantity of conductive plastic which is required can be reduced by a quantity corresponding to the core material. Thus, the quantity of conductive plastic and accordingly the quantity of the expensive carbon black which is required to form one disc can be reduced, and there is an advantage in that the manufacturing cost of the disc can be reduced.
However, according to the disc forming method proposed in the above Laid-Open Application, bubbles are easily mixed between the sheet and the core material when the stampers subject the sheets and the core material interposed therebetween to the compression molding so as to obtain the disc. Further, dust particles or the like easily adheres to the core material when supplying the core material between the sheets, and the dust particles or the like are also subjected to the compression molding. In this case, there is a disadvantage in that the bubbles and foreign substances such as the dust particles remain between the sheet and the core material of the disc which is formed by the compression molding. Such a disc contains irregularities on the signal recording surface thereof, and the recorded signals cannot be reproduced in a normal manner at parts of the recording surface containing the irregularities. In addition, when forming the disc by the compression molding, the sheets and the core material must be supplied to the pressing apparatus independently. However, it is difficult to supply the sheets and the core material independently in a satisfactory manner, and the construction of an apparatus for independently supplying the sheets and the core material to the pressing apparatus is complex. Moreover, it takes a certain time to complete the pressing cycle, and there is a disadvantage in that virtually no measure can be taken to reduce the pressing cycle. Further, when the stampers form the disc by the compression molding, the stampers must press and spread the core material indirectly, that is, through the sheets. For this reason, much of the pressing force of the stampers is used up to spread the core material, and there is a disadvantage in that the variations of geometircal configurations such as the formation of pits cannot be formed on the recording surface of the sheet in a satisfactory manner.